In the naming of a fuel starting with the letter E, this letter denotes ethanol, and the number following the letter E denotes the percentage of ethanol in the mixture comprising substantially petrol and ethanol. The present invention relates to fuels comprising from 10 to 100% ethanol. A fuel E0 does not comprise ethanol.
The phasing or synchronization of the engine consists of determining, during the procedure of starting the engine from the stop thereof, the position of the crankshaft in the cycle of the engine so as to be able to inject fuel as early as possible into the cylinders during the starting of the engine, in synchronization with the progression of the engine cycles. This phasing is performed in a known manner by the engine control unit by means of a position sensor of the crankshaft and/or of the camshaft(s). With some position sensors, the engine control unit may lose the position of the crankshaft when it stops, causing alternating rotary movements before a complete halt, and has to perform this phasing with each starting process. As soon as the phasing has been performed, the engine control unit knows the position of the pistons in the engine cycle, and in particular knows if a piston is in an intake or power phase for the same 360° modulo position of the crankshaft. Once phasing has been performed, and a fuel pressure threshold has been reached, the engine control unit allows the injection controller to inject fuel into the cylinders. The first operational cycle of the engine is that which immediately follows the phasing of the engine and the establishment of a suitable fuel pressure, in which an injection of fuel and identified ignition in the engine cycle are possible.
Ethanol is a fuel which evaporates with greater difficulty than petrol, particularly at cold temperatures. Petrol has a rate of evaporation that is progressive in accordance with temperature, and has a rate of evaporation not equal to zero at low temperature, however this rate is not high. Ethanol behaves very differently and evaporates substantially instantaneously and completely (in a proportion of approximately 99%) at a temperature of approximately 78° C. and at a pressure of approximately 100 Kpa, and thus has a rate of evaporation of substantially zero below this temperature for the same pressure. The greater the rate of evaporation of the fuel, the more effective is the combustion of the gas in the combustion chamber. The greater the proportion of ethanol in the fuel mixture, the slower is the development of the flame front. Thus, the greater the proportion of ethanol in the fuel, the greater are the difficulties encountered when starting the engine, these difficulties being manifested in particular by increased starting times.
Some car manufacturers enforce maximum starting times conditional on the engine temperature at the moment of starting, that is to say the time that passes between switching on the starter and the driving of the engine, or more specifically of the drive shaft or crankshaft in the case of a reciprocating engine, by means of the combustion energy of the mixture in the cylinders for a given temperature or a given temperature range. These starting times may be between 1.5 and 2.5 seconds for example for a fuel E100 at the same temperature.
It is thus necessary to provide specific devices or methods for reducing the starting time of engines using such fuels, more specifically at low temperatures.
The prior art teaches a number of techniques for facilitating cold starting and thus reducing the starting time of engines operating with such fuels.
For example, a technique consisting of adding an additional petrol tank to the vehicle is known, thus solving the problem posed by the increase of the starting time caused by the ethanol component in the fuel. However, such a solution is costly and complicated to implement and also requires an additional volume for housing the additional tank.
A solution consisting of reheating the fuel as it passes into the injectors before entering the combustion chambers is also known. This solution is also costly since it requires specific injectors and an additional consumption of electrical energy.
Other solutions consist of heating the engine block or the common injection rail where applicable. These solutions are also costly since they require expensive additional equipment.
Document EP 1 013 923 is known and describes a method for heating the engine in cold starting conditions. When determined cold starting conditions are confirmed, the ignition timing is advanced beyond the minimum spark advance for best torque by a determined corrective amount so as to facilitate or improve the heating of the engine coolant. This teaching is based on the heating of an engine that turns in steady state under determined engine temperature conditions and indicates that the temperature of the engine coolant rises with an increase of the ignition timing advance relative to the basic ignition advance conditions defined by the minimum advance for the purpose of obtaining the best torque.
A method for controlling the ignition of a combustion engine fed by a fuel containing ethanol, making it possible to overcome the “backfire” problem, is also known from document FR 2 922 965. The “backfire” is the result of an abnormal combustion detonating pre-mix gases inside the intake conduit with a flame still present inside the cylinder. The method according to this document defines a first mode of cold operation of the engine, in which the moment of an ignition step is determined, in a compression phase, by a first advance value with respect to the top dead center greater than a second advance value corresponding to the minimum advance making it possible to produce the maximum engine torque. This first advance value is greater than the second advance value from 1° to 20° of crankshaft rotation, and is lower than a third advance value producing an engine torque equal to 50% of the maximum engine torque. The objective of the over-advance in this document is therefore to initiate combustion earlier in the compression phase so as to avoid the risk of very late combustion, lasting beyond the opening of the intake valves, without increasing either the consumption or polluting emissions of the engine, this over-advance producing an engine torque between 50% and 100% of the maximum engine torque, the method being suitable for an engine operating in the steady state, established while the temperature is below a threshold temperature.